System for cell growth

ABSTRACT

An implantable infection shield and system for drug delivery in vascular tissue includes a relatively non-biodegradable porous linked fibrous biomaterial which controls and directs cell growth and angiogenesis from adjacent vascular tissue into the implant. Infection shield embodiments stimulate cell growth and angiogenesis from adjacent vascular tissue which effectively blocks passage of pathogenic microorganisms along percutaneously implanted objects. In embodiments for drug delivery, a reservoir of the same biomaterial may contain either (1) a cell culture system enclosed within a porous sealable interior chamber or (2) a biodegradable matrix in which one or more drugs are dispersed. After implantation of a reservoir of the first embodiment in an organism, cultured cells obtain food and oxygen via diffusion in tissue fluid through the porous walls of the interior chamber, while metabolic products, including drugs, diffuse away from the cell culture in an analogous manner. In a reservoir of the second embodiment, a biodegradable matrix substantially fills the pores (voids), and progressive dissolution of the matrix releases one or more drugs into surrounding tissue fluid. Reservoirs of either embodiment comprise a plurality of voids of a predetermined size effective for stimulating angiogenesis from the surrounding vascular tissue into at least a portion of the reservoir. The reservoir thus acts to couple a source of drugs to the circulatory system of the organism.

The present application is a Continuation of Prior application Ser. No.08/569,107 filed Mar. 18, 1996 now U.S. Pat. No. 5,964,745, which is acontinuation of U.S. application Ser. No. 08/087,615, filed Jul. 2, 1993and now abandoned.

BACKGROUND Field of the Invention

The invention relates to methods and apparatus for control of cellgrowth, including angiogenesis, in porous implants, produced fromceramics, of the low density type from the general family described inBanas, et al, Thermophysical and Mechanical Properties of the HTP Familyof Rigid Ceramic Insulation Materials, AIAA 20th TermophysicsConference, Jun. 19-21, 1985, Williamsburg, Va. (incorporated herein byreference), Creedon, et al., Strength and Composites, SAMPE Quarterly,October and (incorporated herein by reference), U.S. Pat. No. 4,148,962,issued to Leiser, et al. on April 1979 (incorporated herein byreference). As an example of the general family, a thermal insulationmaterial is produced by Lockheed Missiles & Space Company, Inc. ofSunnyvale, Calif., having the following properties, according to what isbelieved to be an Occupational Health and Safety Administration MaterialData Sheet of Feb. 28, 1989, as follows:

I. PRODUCT IDENTIFICATION

Trade name (as labeled): HTP (High Thermal Performance) MaterialChemical names, common names: Thermal insulation material.

Manufacturer's name: Lockheed Missiles & Space Company, Inc.

Address: 1111 Lockheed Way, Sunnyvale, Calif. 94089

Emergency phone: (408) 742-7215 Refer questions to: (6 .a.m.-5 p.m. PST)Lockheed Missiles & Space (408) 742-3536 Company, Inc. (Off Hours)Occupational Safety & Business phone: (408) 742-7215 Health Dept.Org/4720 - 8/106 Date prepared: 1/89

II. HAZARDOUS INGREDIENT

Exposure Limits in Air Chemical Gas Percent AGGIH Names Numbers (by vt.)OSHA (PEL) (TLV) Other Alumina 1344-28-1 10-50 5 mg/m³/ 10 mg/m³ Fiber15 mg/m³ (Total (Respirable/ nuisance Total dust) dust) Silica60676-86-0 50-90 5 mg/m³/ 10 mg/m³ See Fiber 15 mg/m³ (Fibrous Health(Respirable/ Glass) Effect Total dust) Silicon 409-21-2 1-3 5 mg/m³/ 10mg/m³ Carbide 15 mg/m³ (Total (Respirable/ nuisance Total dust) dust)Boron 10-043-115 1-5 5 mg/m³/ 10 mg/m³ Nitride 15 mg/m³ (Total(Respirable/ nuisance Total dust) dust)

III. PHYSICAL PROPERTIES

Vapor density (air=1): NA

Specific gravity: Varies

Solubility in water: Nil

Vapor pressure, mmHg at 20 degrees c: NA

Evaporation rate (butyl acetate=1): NA

Appearance and odor: Solid off-white blocks, no odor.

Softening point or range, degrees F: 2876

Boiling point or range, degrees F: NA

IV. FIRE AND EXPLOSION

Flash Point, degrees F: Nonflammable (will not support combustion)

Autoignition temperature, degrees F: NA

Flammable limits in air, volume %: NA

Fire extinguishing materials: NA

Special firefighting procedures: NA

Unusual fire and explosion hazards: NA

V. HEALTH HAZARD INFORMATION Symptoms of Overexposure

Inhaled: Irritation or soreness in throat and nose. In extreme exposuresome congestion may occur.

Contact with skin or eyes: Local irritation, rash.

Swallowed: Not a primary entry route.

Health Effects or Risks from Exposure

Acute: Mechanical irritant to skin, eyes, and upper respiratory system.

Chronic: Results of studies on the effect of silica fiber exposurecausing malignant and non-malignant respiratory disease in man arecontroversial. Studies on laboratory animals fall in to two categories:animals which breathed high concentrations showed no disease, while someexposed through artificial means (e.g., implantation) have developedcancer. Recent U.S. and European studies of almost 27,000 productionworkers. (1930s to 1980s) found no significant increase in disease fromfiber glass exposure. Even though the extensive human studies werejudged inadequate for carcinogenicity, IARC has classified glass wool aspossibly carcinogenic for humans, based on the artificially exposedanimal studies. Fibrous glass is not considered a carcinogen by NTP andOSHA. As a conservative approach in the absence of conclusive knowledgeindicating otherwise, we recommend treating this material as if it is apotential carcinogen. Handling procedures such as HEPA vacuum and localexhaust ventilation should be used to minimize exposure. See SpecialHandling Procedures.

Periodic air monitoring is recommended. The NIOSH recommended exposurelimit for fibrous glass is 3 fibers/cc. The manufacturer of the silicafiber used in this product recommends an exposure limit of 1 fiber/cc.

First Aid

Skin: Wash thoroughly with soap and water.

Eyes: Flush thoroughly with water for 15 minutes.

Inhaled: Move person to fresh air at once. If person has stoppedbreathing, administered artificial respiration. Get immediate medicalattention.

Suspected Cancer Agent

No.: This product's ingredients are not found in the lists below.

_X_ Yes: _(———) Federal OSHA _(———) NTP _X_ IARC

Medical Conditions Aggravated by Exposure

Pre-existing upper respiratory conditions and lungs diseases may beaggravated.

VI. REACTIVITY DATA

Stability: _X_ Stable _(———) Unstable

Incompatibility (materials to avoid): Will react with hydrofluoricacide.

Hazardous decomposition products: NA

Hazardous polymerization: _(———) May occur _X_ Will not occur

Conditions to avoid: None

VII. SPILL LEAK AND DISPOSAL PROCEDURES

Spill response procedures: Wet down spills to control dust. Material isnot considered a hazardous waste under 40 CFR. Dispose of all wastes inaccordance with federal, state and local regulations.

VIII. SPECIAL HANDLING INFORMATION

Ventilation and engineering controls: Local exhaust ventilation shouldbe used for grinding or other operations which generate dust. Hoodexhaust should be fitted with a filter which will control 99% of fibersless than 1 micron in diameter.

Respiratory protection: For exposures up to 10 f/cc, use aNIOSH-approved twin cartridge air purifying respirator with highefficiency particulate air (HEPA) filters. For exposures up to 50 f/cc,use a NIOSH-approved full-face respirator with HEPA filters. Above theselevels, use an air-supplied respirator.

Eye protection: Safety glasses with side shields should be worn ifmaterial is ground, cut, or otherwise disturbed using power tools.

Gloves: Any barrier material.

Other clothing and equipment: Wear loose fitting, long sleeved clothing;Wash exposed areas with soap and warm water after handling; Wash workclothes separately from other clothing; rinse water thoroughly.

Other handling and storage requirements: Protect against physicalhandling damage.

Protective measures during maintenance of contaminated equipment: Wear arespirator as prescribed in the Respiratory Protection section. Weargloves and coveralls as appropriate to prevent skin contact.

IX. LABELING

Labeling: Fibrous glass-type materials. Treat as a potential carcinogen.

Acute: May cause skin, eye and respiratory tract irritation.

Chronic: Long term inhalation may cause serious respiratory disease.Handle wet and use respiratory protection.

Proper Shipping Name: Not regulated.

Also, a reusable surface insulation (HRSI) is described by a LockheedMissles & Space Co. Fact Sheet released September, 1988, titled ThermalProtection System (incorporated herein by reference)

Cell Growth in Implants

Implants for drug delivery and infection control preferably interactwith the organism in which they are implanted, the interaction beingthrough the medium of tissue fluid and by cellular contact with theimplant. The extent of angiogenesis and cellular growth within theimplant and the distance through which materials from the implant mustdiffuse through the tissue fluid to reach the organism's circulatorysystem may have important effects on functioning of the implant. Thelatter parameter is especially applicable in the case of implants fordrug delivery.

Applications for Drug Delivery Implants

Administration of one or more drugs to a patient at predetermined dosagerates is required for effective treatment and/or prevention of severalinfectious diseases, including, e.g., tuberculosis, malaria and certainsexually-transmitted diseases (STD's). Public health measures adopted tocope with these diseases rely heavily on administration of prophylacticand treatment drugs on an outpatient basis, but the rising incidence andprevalence of infectious diseases in certain populations (e.g., homelessor medically indigent families and migrant workers) reflect the limitedefficacy of current treatment and prevention programs in such groups.

The success of outpatient treatment and prevention programs dependssubstantially on each patient's compliance with prescribed dosage(s) toachieve and maintain therapeutic or prophylactic drug levels. Deviationfrom a predetermined dosage rate or duration may result in a relapse orexacerbation of the disease at issue. In particular, a patient'spremature termination of orally administered drug treatment can allowthe survival and proliferation of drug-resistant microorganisms, as hasoccurred patients having tuberculosis and STD's such as gonococcalsalpingitis.

Patients infected with relatively drug-resistant pathogens becomeprogressively more difficult and expensive to treat. Those not treatedor inadequately treated act as reservoirs of disease. They easily infector reinfect those with whom they have contact, and thus constitute asignificant public health threat. Especially within transientpopulations and those living in crowded public accommodations,infectious diseases will continue to be passed back-and-forth unless thechain of transmission is broken through effective treatment ofinfectious patients. One means of providing such treatment involvesproviding effective drug therapy through systems for controlled ordelayed drug release in vivo. In patients who present repeatedly withthe same disease and who either can not or will not comply with an oraldosage regimen, implants which operate automatically to providetherapeutic drug levels in vivo may reasonably be offered as part ofeffective therapy.

Existing Implantable Drug Delivery Systems

A variety of implantable drug delivery systems already exist forcontrolled release of drugs in vivo over prescribed periods of time.Examples include: (1) systems comprising drugs encapsulated innon-biodegradable membranes, e.g., levonorgestrel in flexible closedcapsules made of SILASTlC® brand dimethylsiloxane/methylvinylsiloxanecopolymer (the NORPLANT® system); (2) drugs prepared in relativelyinsoluble form for intramuscular, intra-articular or subcutaneousinjection, e.g., penicillin G benzathine and penicillin G procaine(BICILLIN® C-R), methylprednisolone acetate aqueous suspension(DEPO-MEDROL®), or norethisterone dispersed in poly(DLlactide-co-glycolide) microcapsules; and (3) drugs dispersed in formedbiodegradable implants, the implants comprising, e.g.,polyhydroxybutyrate with or without hydroxyapatite. All of thesesystems, however, are associated with significant disadvantages.

Encapsulated drug forms intended for implantation, as in the NORPLANT®system, are subject to errors in placement which may cause capsuleexpulsion and consequent irregularities in drug delivery rate. Capsulesmay also be difficult to remove, but can not be left in placeindefinitely (it is recommended that all capsules be removed after fiveyears).

Additionally, intramuscular, intra-articular or subcutaneous injectionsof drugs such as BICILLIN® C-R, DEPO-MEDROL® or norethisterone arepainful, and patients may tend to delay or avoid treatments involvingrepeated injections due to the expected discomfort. Furthermore, therate of drug delivery from subcutaneous dosage forms is substantiallylimited by the local blood supply.

Finally, formed implants of biodegradable polyester, even whenreinforced with hydroxyapatite, tend to experience significant declinesin elastic modulus and bend strength after weeks to months ofimplantation. The resulting tearing and cracking of the implant can thenalter the amount of implant surface exposed to body fluids and cellularactivity, which in turn may cause unpredictable changes in the deliveryrate of any drug(s) dispersed within the implant. Because stability andpredictability of drug administration rates are paramountconsiderations, implants containing brittle materials or drug depositsshould ideally retain their shape and strength during the entire courseof implantation and even after depletion of the administered drug. Highlevels of shape and strength retention would also facilitate changes inthe drug administration regimen and would also allow removal of theimplant at the convenience of the patient rather than on a fixedschedule.

In view of the disadvantages summarized above for currently availableimplantable drug delivery systems, a more flexible and reliableimplantable system for drug delivery is needed. Changes in the drugtreatment regimen (i.e., drug selection and dosage rate) should berelatively easily made and easily changed, and communication between theimplant and the local tissue into which it is implanted should becontrollable throughout the life of the implant through selectivestimulation of cell growth and angiogenesis from the local tissue to theimplant. At the present time, no system combining these desiredcharacteristics is commercially available.

Implants for Infection Control

The functions described above as useful in implants for drug deliverywould also be useful in implants for infection control, as where a breakoccurs in the skin at a site of percutaneous catheter insertion.Implants currently used in such applications frequently comprise one ormore fibrous cuffs for interface with the body tissues, but the cuffsthemselves may become infected because normal immune responses areimpeded in the area of the implant. Catheter-related infections may thusbe reduced by improved communication between the cuff implant and thelocal tissue. As in the case of implants for drug delivery, functionalintegration of the cuff implant with surrounding tissue would preferablybe controllable throughout the life of the implant by selectivestimulation of cell growth and angiogenesis from the local tissue to theimplant. Implantable cuffs facilitating such control are not, however,commercially available.

SUMMARY OF THE INVENTION

Implants for drug delivery and infection control (infection shields)according to the present invention substantially avoid the shortcomingsof prior implants noted above by incorporating an implantable system forcell growth control as described herein. Each drug delivery implant ofthe present invention comprises a porous linked fibrous biomaterial drugreservoir, the voids of which, in some embodiments, contain one or moredrugs which may be dispersed within a biodegradable matrix. Cell growthand angiogenesis within the reservoir is controlled and directed asdescribed herein. Note that drugs to be delivered, as well as the matrixmaterials (if present), may include metabolic products of the organismin which a drug reservoir is intended to be placed, or of otherorganisms.

In other embodiments, a cuff-shaped infection shield inhibits thepassage of pathogenic microorganisms along a catheter or otherpercutaneously implanted device through control of cell growth andangiogenesis within the shield as described herein. Further embodimentsinclude a reservoir having one or more sealable interior chamberscontaining cultured living cells which can communicate through porouschamber walls, by the medium of tissue fluid and/or cell growth medium,with cells of the organism in which the reservoir may be implanted orwith an external fluid exchange system (as in a bioreactor). Infectionshielding cuffs or reservoir implants according to the present inventionboth comprise fibrous biomaterials which are biocompatible. As describedherein, biocompatible implants support controlled cell growth andangiogenesis within an organism while not evoking a foreign body immuneresponse which significantly adversely affects preferred implantfunction. Implant biomaterials may be biodegradable (i.e., they maydissolve in tissue fluid to form nontoxic solutions), or they may besubstantially non-biodegradable (e.g., silica fibers).

Implantation of infection shields and drug reservoirs of the presentinvention is preferably carried out in vascular tissue of an organism.Vascular tissue is tissue which contains circulatory system vessels(including lymphatic and blood vessels) and tissue fluid in sufficientquantity to sustain cells growing within the implant and to transportdrug released from a reservoir implant to the circulatory systemvessels.

Drug transport may be by diffusion, convection, or facilitateddiffusion. In reservoirs which contain cell cultures and are implantedwithin vascular tissue, food and oxygen diffuse toward the culturedcells and metabolic products (including one or more desired drugs)diffuse away from them via the tissue fluid. Similarly, cells invadingthe implant from the local tissue of the organism are sustained throughexchange of food, oxygen and metabolic products with circulatory systemvessels growing within the implant from the local tissue.

In all embodiments of the present invention, a reservoir or infectionshield implanted in vascular tissue tends to: (1) retain the desiredimplant shape and structural integrity for a duration of implantationwhich substantially exceeds the planned duration of implantation for theshield or the duration of drug administration from a reservoir implant,and (2) aid in sustaining cells growing within the implant and/orcoupling drugs emanating from the reservoir to the circulatory systemfor timely delivery of effective drug doses to one or more desired sitesof action within the organism. Each implant embodiment reliably performsthese functions over periods of implantation from a few days to severalmonths, depending on its design. Note that the tendency for embodimentsof the present invention to retain a desired implant shape does notpreclude flexible implants according to the present invention (e.g.,implants in the form of a flexible sheet). In such implants, flexibilitydoes not substantially degrade the functions of stimulation of cellgrowth and angiogenesis, and/or support of cultured cells within theimplant.

Drug reservoirs and infection shields in all embodiments of the presentinvention comprise relatively non-biodegradable fibrous biomaterialslinked at fiber intersections to aid in substantially retaining theirshape after prolonged implantation. Shape retention includes retentionof the mechanical integrity of any cell culture or biodegradable matrixwhich may be present, i.e., substantial disruption of the cell spacingand matrix fragmentation are avoided for at least the useful life of theimplant. The fiber linking which facilitates shape retention includesprocesses capable of substantially maintaining the spatial relationshipof one fiber with respect to other fibers which touch it for theeffective life of an implant comprising fibers. Process examples includefusing (e.g., with silica fibers), chemical bonding (e.g., with polymerfibers), and adhesion (e.g., with colloidal silica). Additionally, andnotwithstanding their relatively non-biodegradable porous linked fibrousbiomaterial component, reservoirs and infection shields of the presentinvention are substantially biocompatible.

Angiogenesis in the Implant

In particular, implant biocompatibility is reflected both in the abilityto stimulate and sustain populations of cells within the implants, andin the function of coupling drugs which emanate from within a reservoirto the circulatory system of the organism in which the reservoir isimplanted. Within portions of implants intended to either sustaincultured cells or stimulate angiogenesis or cell growth from adjacentvascular tissue, the implant material is substantially hydrophilic andcontains mean pore (void) sizes and porosities which have beenempirically determined to support the desired function of the implant.

Angiogenesis within the implant helps ensure that it is functionallyintegrated within the circulatory system of the patient into which theimplant is placed. The controlled and progressive nature of angiogenesisand cell growth into implants differentiates implantable infectionshields and systems for drug delivery of the present invention from allprior devices, systems and methods.

Cell growth in general and angiogenesis in particular within implants ofthe present invention is a function of the mean void size, fibercomposition and surface chemical characteristics of the biomaterialfibers. In implants comprising fibrous biomaterials of substantiallyuniform fiber size range and fiber distribution, implant density issubstantially inversely related to void or pore size. For example,implants comprising Q-Fiber® (amorphous high purity silica) obtainedfrom the Manville Division of Schuller International, Inc., Waterton,Ohio, and prepared as described herein at high density (39 pounds/cubicfoot) support approximately ⅓ the cell growth of similar materialprepared at a low density of 12 pounds/cubic foot). Hence, areas of highand low cell growth potential may be incorporated in an implant bymaking the respective portions of low and high density material. Toachieve the desired ratio of high/low cell growth potential, one needonly perform in vitro tests using cells of the tissue in whichimplantation is desired or cells of the type desired to be cultured.Preferred high and low density values for sections of an implant whichare to respectively inhibit or support cell growth may thus bedetermined. Note that in other preferred embodiments of the presentinvention, conditions of high and low cell growth potential may beachieved at least in part by alterations in fiber surface compositionand/or coatings, in addition to or in place of density alterations.

In any embodiment of the present invention, it is preferable thatinitial implant densities (ignoring any matrix which may be present)remain substantially unchanged throughout the useful life of theimplant. Such consistency of density may be achieved through linking ofthe biomaterial fibers comprising the implant. Linking acts to maintainthe range of void or pore sizes necessary for proper functioning of theimplant. The degree of linking and the degree of flexibility atindividual linkages required will be empirical functions of the fibertype chosen and the strength requirements of the particular implantconfiguration chosen (e.g., elastic modulus, bending strength).

Composition and Function of a Biodegradable Matrix

Within reservoirs of certain preferred embodiments of the presentinvention, one or more treatment or prophylaxis drugs are dispersedwithin a matrix, the matrix being dispersed within the pores (voids) ofthe linked fibrous biomaterial. The matrix comprises one or morebiodegradable biomaterials, the exact composition being determined bythe desired rate and duration of matrix biodegradation (with itsresultant drug release). Note that drugs may be microencapsulated priorto dispersion within the matrix to further delay their release in activeform and/or to reduce the concentration of free drug in the immediatevicinity of the reservoir.

Suitable materials for the biodegradable matrix include but are notlimited to homopolymers (e.g., poly-paradioxanone, polylysine orpolyglycolic acid) and copolymers (e.g., polylactic acid andpolyglycolic acid). Biodegradable polymers may be augmented in thematrix (or even replaced, in certain embodiments) by other biodegradablebiomaterials, including but not limited to, e.g., Glassfiber®, plasterof Paris, beta-whitlockite, hydroxyapatite, and various other calciumphosphate ceramics.

Structure and Function of the Implant

In all embodiments of the present invention comprising a matrix, theporous linked fibrous biomaterial tends to establish and maintain thephysical characteristics of the implant (and any matrix or drugcontained therein), and to direct newly-formed blood vessels thereto,i.e., acting to control the number and location of the newly-formedblood vessels within the implant. Implants which contain a biodegradablematrix acquire new and/or larger voids as the matrix is removed throughthe action of tissue fluid. Thus, there is within the implant a changinglevel and location of angiogenesis and new cell growth as portions ofthe matrix are biodegraded.

Such local direction of cell growth and blood vessel proliferationeffectively controls and directs the implant integration andbiodegradation processes. Similarly, the rate of absorption of cellculture metabolic products in implants containing cultured cells is alsoregulated.

For embodiments employing cell cultures, diffusion distance from thecultured cells to the circulatory system may remain substantiallyunchanged after initial angiogenesis within the reservoir. For drug ormatrix-containing embodiments on the other hand, the diffusion distancefrom matrix to circulatory system will in general be constantlychanging.

If desired, the effective mean distance over which matrix components(including drugs) must diffuse to reach the circulatory system can bemaintained substantially constant throughout the life of the presentimplant. As matrix components are dissolved and carried away by firstthe tissue fluid and then the blood stream, angiogenesis results in theeffective repositioning of the circulatory system closer to theremaining (undissolved) matrix. Angiogenesis, in turn, is controlled byseveral factors including, but not limited to: void size in thereservoir, reservoir porosity, and the composition of the reservoir'slinked fibrous biomaterial.

Angiogenesis may be encouraged or inhibited at a particular locationwithin the implant because it is effectively directed by thecommunicating voids of the implant only if the voids are within anempirically predetermined preferred size range. Voids either too largeor too small will substantially inhibit or even prevent angiogenesis. Onthe other hand, voids within a preferable size range will stimulateextension of the circulatory system with the implant.

In embodiments of the present invention having a biodegradable matrix,new voids are formed continuously by dissolution of the biodegradablematrix; actual void size progressively increases toward the limitallowed by the reservoir structure. In contrast, in embodiments withouta matrix, the voids present initially on implantation are thosecharacteristic of the linked fibrous biomaterial. In either type ofembodiment, however, drug absorption by the circulatory system can bemade to proceed in an orderly and substantially predictable manner.

In matrix-containing embodiments, drug absorption at substantiallypredetermined rates occurs even as the shape and size of thebiodegradable matrix mass changes. No prior drug delivery systemsoperate in this manner to ensure a controlled blood flow adjacent to adrug-producing or drug-storing implant, even when a drug storage element(i.e., the matrix) is itself a changing biodegradable moiety.

Effects of Implant Porosity

Note that while voids of the proper size will tend to stimulateangiogenesis in certain areas of the implant, the porosity of the linkedfibrous biomaterial will ultimately limit the total blood flow per unitvolume of the reservoir. Porosity is defined as the percent of voidspace relative to a given volume of linked fibrous biomaterial materialin the implant (ignoring any matrix which may be present). Because anincrease in porosity tends to allow an increase in the total amount ofblood flow in the implant (through angiogenesis), it also tends todecrease the mean diffusion distance separating blood vessels fromcultured cells or biodegradable matrix components within the implant.

Conversely, decreasing the porosity of the reservoir tends to increasethe mean diffusion distance. Thus, the choice of preferred porosity forany reservoir (or portion thereof) will depend on the desired density oftissue ingrowth or the flux of drug desired from the implant. Forexample, relatively high drug flux values would ordinarily be desirablefor implants delivering antibiotics, while relatively low drug fluxvalues would be needed for delivery of hormones.

Note that the amount of drug flux needed from a given implant may beinfluenced by placement of the implant. Proper choice of an implantationsite may result in relatively higher drug concentrations in certainregions of the body where the drug is most needed, thus perhaps allowinglower average blood levels of the drug.

Cell Isolation by Channel Size Control

An important aspect of the structure of infection shield implants andreservoir implants intended to contain cultured cells is the presence ofchannels for tissue fluid which are too small for cell or vessel growthbut large enough to allow effective diffusion of food, oxygen andmetabolic products between cells and vessels. Such channels caneffectively isolate interior portions of a reservoir from contact withthe host organism except through the medium of tissue fluid componentswhich pass through them. Similarly, such channels can inhibit thepassage of microorganisms through an infection shield while supportinggrowth of skin or subcutaneous tissue into other portions of the shield.

In preferred embodiments of the present invention containing culturedcells, channels within portions of the reservoir intended to supportangiogenesis from the host organism will adjoin the porous wall of asealable inner chamber of relatively dense porous linked fibrousbiomaterial. The chamber wall, which is preferably relatively thin,effectively separates host organism cells and new blood vessels formedin the reservoir from the cultured cells, except for communicationthrough tissue fluid channels in the porous chamber wall. Mechanicalsupport for the thin chamber wall is provided by linked biomaterialfibers which, in a less-dense linked pattern, comprise the remainingstructure of the reservoir. Preferred density ranges for each reservoirmaterial and cultured cell type are empirically determined by in vitrotesting.

Implant Placement

Infection shield embodiments of the present invention are preferablyplaced around a percutaneously-placed object (e.g., a catheter) at ornear the point where the object passes through the skin and subcutaneoustissue. In certain embodiments, the shield comprises a substantiallycylindrically shaped catheter seal for substantially circumferentiallysurrounding the catheter, the seal comprising porous linked fibrousbiomaterial (e.g., silica fiber) having a plurality of voids ofpredetermined mean void size effective for inhibiting angiogenesis fromthe skin and subcutaneous tissue, and a tissue cuff circumferentiallysurrounding the catheter seal, the cuff comprising porous linked fibrousbiomaterial having a plurality of voids of a predetermined mean voidsize effective for stimulating angiogenesis in the cuff from the skinand subcutaneous tissue.

Tissue ingrowth with attendant angiogenesis links the skin andsubcutaneous tissue with the implant. Such ingrowth is preferablystopped adjacent to the object by a layer of relatively dense linkedfibrous biomaterial which substantially blocks further tissue ingrowthand angiogenesis, but which does not provoke a foreign body responsefrom the organism in which the shield is implanted. Thus, passage ofpathogenic organisms around the percutaneously-placed object iseffectively blocked by the ingrowth of tissue, and infection isprevented. This type of implant placement differs substantially fromthat preferred for drug delivery systems.

Drug delivery implants of the present invention are preferably placedwithin or adjacent to vascular tissue. Such tissue offers an appropriatebase from which angiogenesis from the tissue to within the reservoir(characteristic of all embodiments after implantation) may proceed. Twopreferred locations for implantation are within the marrow of long bonesand within a surgically constructed peritoneal pouch.

One alternative method of reservoir placement within bone is to securethe reservoir itself with external fixation in a manner similar to thatused for fixation of bone fractures. Reservoirs of the present inventionmay be shaped to fill existing bony defects (e.g., missing bone due toinjury), and one or more drugs to stimulate osteogenesis (e.g.,transforming growth factor beta, osteogenin and osteocalcin) may, forexample, be dispersed within the matrix. Reservoirs for applicationsrequiring external fixation would typically comprise relativelyhigh-strength material fibers and relatively high levels of linking atfiber crossings. Such reservoirs would biodegrade relatively slowly overtime as the strength required of the reservoir is increasingly providedby newly formed bone within the voids of the reservoir.

Another alternative method of reservoir implantation in bone requiresinstallation of a permanent fixture within the bone. The fixture allowsready access to the implant and frequent, substantially atraumaticchanges of the reservoir. The general design of such a fixture issuggested by reference number 12 in U.S. Pat. No. 4,936,851 (Fox etal.), incorporated herein by reference. A fixture of this general designcan be allowed to become a substantially permanent part of the bone inwhich it is placed (using methods for implantation and subsequent woundcare similar to those described in Fox, et al.).

After implantation, the fixture may accommodate one or moresubstantially cylindrically shaped reservoirs of the present invention.Properly-shaped fenestrations in the fixture wall (see reference number15 of Fox, et al. for an example of one type of fenestration) allowangiogenesis of the porous linked fibrous biomaterial of thereservoir(s). Note that stacking of two or more substantiallycylindrical reservoirs in a fixture implanted in bone is a preferredmethod of simultaneously providing more than one drug, or providing asingle drug having more than one desired flux level over time, to anorganism by using devices of the present invention. A desired ratio ofthe drugs provided may be easily achieved through appropriate choice ofthe lengths and/or drug release capacities of reservoirs inserted in thefixture. Similarly, drug combinations and ratios are easily changedthrough replacement of an existing set of implanted reservoirs (orportions thereof) with another set.

Note also that substantially cylindrical reservoirs for delivery ofdifferent drugs, or for delivery of the same drug at different rates,can be cut into a variety of differing forms, with pieces from differentreservoirs being reassembled into a substantially cylindrical formsuitable for insertion into a fixture. Such a mosaic reservoir mayprovide a variety of drug dosage profiles over time, as may be requiredin certain drug treatment and prophylaxis protocols.

Access to the fixture through a small skin incision and a fixture cap(see reference number 14 of Fox, et al. for an example of one type offixture cap) could be substantially as described in Fox et al. Removalof a cylindrically shaped reservoir from a fixture in which it hasbecome substantially integrated with both the bone tissue andcirculatory systems of the bone marrow may be accomplished through aprocess analogous to trephination. Fox, et al does not describe aseparate trephine tool, but methods and devices for trephination arewell known to those skilled in orthopedics and neurosurgery.

For implants of the present invention having a longer projected life, orthose in which implantation in the abdominal cavity is desired,implantation of reservoirs in a peritoneal pouch created by open orendoscopic surgery may be desirable. By totally enclosing each implantin a peritoneal cover, substantial potential for angiogenesis isprovided, while the likelihood of adhesion formation between theexternal surface of the pouch and adjacent structures is minimized.

Preparation of Porous Linked Fibrous Biomaterial Reservoirs

Porous linked fibrous biomaterial reservoirs of the present invention donot have a fixed composition. They are relatively non-biodegradable forthe functional life of the implant, retaining sufficient mechanicalstrength to maintain porosity values and void size consistent with thedegree of angiogenesis desired in the reservoir. In certain preferredembodiments (e.g., for insertion in and subsequent removal from fixturesin bone), they preferably comprise nonwoven, randomly oriented,high-purity silica fibers which are linked at a plurality of crossingpoints into a substantially non-biodegradable porous structure. In otherpreferred embodiments (e.g., for one-time delayed-release drugadministration), a reservoir may preferably comprise linked Glassfiber®which will retain its shape until the reservoir drug is exhausted oruntil any cultured cells within the reservoir become non-viable.

A general method for making linked fibrous silica is described in U.S.Pat. No. 3,952,083 (Fletcher, et al.), which is incorporated herein byreference. Alterations of the method of Fletcher, et al. to make theporous linked fibrous biomaterial of the present invention are evidentin the manufacturing protocol provided in the Detailed Description givenbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cylindrically shaped reservoir for insertion in afixture in bone.

FIG. 2 illustrates two cylindrical reservoirs of differing compositionintended for simultaneous insertion in a fixture in bone.

FIG. 3 illustrates a cylindrical reservoir having longitudinalcylindrical segments; each segment may have a different composition.

FIG. 4A illustrates a reservoir suitable for cell culture and forinsertion in a fixture in bone.

FIG. 4B illustrates a void insert of linked fibrous biomaterial intendedfor use with the reservoir of FIG. 4A.

FIG. 5A illustrates a reservoir suitable for cell culture and forimplantation in a peritoneal pouch.

FIG. 5B illustrates a cap assembly for occluding the cell culture cavityof the reservoir of FIG. 5A.

FIG. 5C illustrates a void insert of linked fibrous biomaterial intendedfor use with the reservoir of FIG. 5A.

FIG. 6 illustrates an infection shield applied around a catheter.

FIG. 7 is a block diagram of a process to make linked silica fiberaccording to the present invention.

DETAILED DESCRIPTION

Implantable infection shields and systems for drug delivery according tothe present invention comprise porous linked fibrous biomaterialdisposed to either stimulate or inhibit cellular growth and/orangiogenesis, according to the predetermined requirements of the variousembodiments.

One embodiment is a reservoir which contains within it a source of oneor more drugs to be delivered. Intended for implantation in vasculartissue, the drug source may be a biodegradable matrix in which the drugor drugs to be delivered are dispersed, and which dissolves slowly intissue fluid from the organism in which the reservoir is implanted. Thesource may also be a cell culture contained within a sealable porouschamber within the reservoir. Cultured cells receive food and oxygen bydiffusion in the tissue fluid which passes through the sealable porouschamber walls. Cell-to-cell contact between cells of the organism andcultured cells is, however, prevented.

Thus, a method for making a system for drug delivery for implantation invascular tissue, the method comprises obtaining a reservoir comprisingporous linked fibrous biomaterial having a plurality of voids of apredetermined mean void size effective for stimulating angiogenesis insaid reservoir from the vascular tissue, providing a biodegradablematrix, dispersing a drug to be delivered in said biodegradable matrixto form a drug delivery matrix, and dispersing said drug delivery matrixwithin said voids to make a system for drug delivery.

FIG. 1 illustrates a preferred embodiment of a reservoir 10 comprisingporous linked fibrous biomaterial 12, according to the presentinvention; the reservoir 10 is suitable for insertion into a fixture ina bone analogous to reference number 12 in U.S. Pat. No. 4,936,851 (notshown). The reservoir 10 may also be inserted directly in vasculartissue (e.g., breast tissue), and the reservoir surface area may beincreased by changing its shape (e.g., by flattening it) or byperforating the reservoir 10 with one or more holes 11 or depressions 9.

FIG. 2 illustrates two reservoirs 22,24 similar to the reservoir 10 inFIG. 1, except that they are intended for simultaneous insertion into afixture in a bone (not shown). Together, the two reservoirs 22,24comprise a new reservoir 20 which may serve as the source of twodifferent drugs, reservoir 22 providing one drug and reservoir 24providing the other. Note that the reservoirs 22,24 may also provide thesame drug, but at differing rates and for differing durations.Simultaneous insertion of reservoirs 22,24 then allows the new reservoir20 to provide a drug at a rate which varies with time. Note that in amanner analogous to that shown in FIG. 2, a plurality of drugs may beprovided in fluxes having predetermined ratios to one another throughsimultaneous insertion of appropriate drug reservoirs in one or morefixtures in bone.

FIG. 3 illustrates another form of reservoir 30 which may act as asource for each of the drugs contained within longitudinal cylindricalsegments 31-36. The reservoir 30 may also be inserted in a fixture in abone as noted above (not shown).

FIG. 4A illustrates a reservoir 40 of the present invention intended tocontain a cell culture (not shown) and for coupling the cell culture tovascular tissue (not shown) in which the reservoir may be implanted. Acell culture may be contained within a sealable interior chamber, thewall 44 of which is illustrated. The chamber wall 44 is sealed at end 45but is shown open at end 41. Chamber wall 44 comprises porous linkedfibrous biomaterial having a plurality of voids of a predetermined meanvoid size effective for inhibiting angiogenesis in chamber wall 44 fromthe vascular tissue in which reservoir 40 is intended to be implanted.Culturea cells may be inserted within central void 46 within chamberwall 44, and then sealed therein by inserting plug 49 of cap assembly 48within central void 46. Outer coat 43 comprises porous linked fibrousbiomaterial having a plurality of voids of a predetermined mean voidsize effective for stimulating angiogenesis in reservoir 40 from thevascular tissue in which reservoir 40 is intended to be implanted. Notethat for clarity in FIG. 4A, outer coat 43 is shown cut back fromchamber wall 44. In preferred embodiments of the present invention,outer coat 43 is not cut away as shown in FIG. 4A, but insteadsubstantially completely surrounds chamber wall 44. Note also thatcultured cells (not shown) within central void 46 may preferably grow bylayering on the surface of chamber wall 44 which faces central void 46.Cultured cells may also preferably grow within and on void insert 47(illustrated in FIG. 4B) if insert 47 is placed within void 46 prior tosealing with plug 49 of cap assembly 48. Insert 47 comprises porouslinked fibrous biomaterial having a plurality of voids of apredetermined mean void size effective for stimulating growth and/ordifferentiation of cultured cells.

FIG. 5A illustrates a reservoir 50 which is analogous to reservoir 40 inFIG. 4A except that it provides a larger ratio of area of chamber wall54 to volume of central void 56. Other shapes (not illustrated) forchamber wall 54 might also be chosen for certain embodiments (e.g., asubstantially cubic shape). A reservoir having a shape analogous to thatof reservoir 50 may, for example, be preferred for implantation in aperitoneal pouch. If reservoir 50 is used in a bioreactor application,the reactor would preferably comprise a plurality of reservoirs 50 heldin spaced relationship within surrounding fluid growth medium and/ortissue fluid.

A cell culture may be contained within a sealable inner chamber ofreservoir 50, the wall 54 of which is illustrated. The chamber wall 54is sealed at end 55 but is shown open at end 51. Chamber wall 54comprises porous linked fibrous biomaterial having a plurality of voidsof a predetermined mean void size effective for inhibiting angiogenesisin reservoir 50 from the vascular tissue in which reservoir 50 isintended to be implanted. Chamber wall 54 also acts to prevent culturedcells from passing through the wall 54. Cultured cells may be insertedwithin central void 56 within chamber wall 54, and then sealed thereinby inserting plug 59 of cap assembly 58 (see FIG. 5B) within centralvoid 56. Outer coat 53 comprises porous linked fibrous biomaterialhaving a plurality of voids of a predetermined mean void size effectivefor stimulating angiogenesis in reservoir 50 from the vascular tissue inwhich reservoir 50 is intended to be implanted. Note that in bioreactorapplications, outer coat 53 acts to provide mechanical strength to therelatively thin chamber wall 54. Note also that for clarity in FIG. 5A,outer coat 43 is shown cut back from chamber wall 54. In preferredembodiments of the present invention, outer coat 53 is not cut away asshown in FIG. 5A, but instead substantially completely surrounds chamberwall 54. Note also that cultured cells (not shown) within central void56 may preferably grow by layering on the surface of chamber wall 54which faces central void 56. Cultured cells may also preferably growwithin and on void insert 57 (illustrated in FIG. 4C) if insert 57 isplaced within void 56 prior to sealing with plug 59 of cap assembly 58.Insert 57 comprises porous linked fibrous biomaterial having a pluralityof voids of a predetermined mean void size effective for stimulatinggrowth and/or differentiation of cultured cells.

FIG. 6 illustrates an infection shield 90 for a catheter intended forplacement through skin and subcutaneous tissue according to the presentinvention; shield 90 is shown applied around a catheter 92. Infectionshield 90 comprises a catheter seal 96 and a tissue cuff 94. Catheterseal 96 comprises substantially cylindrically shaped porous linkedfibrous biomaterial (e.g., silica fiber) for substantiallycircumferentially surrounding a catheter, the seal 96 having a pluralityof voids of a predetermined mean void size effective for inhibitingangiogenesis from the vascular tissue which may contact infection shield90. Tissue cuff 94 comprises porous linked fibrous biomaterial (e.g.,silica fiber) having a plurality of voids of a predetermined mean voidsize effective for stimulating angiogenesis in cuff 94 from the skin andsubcutaneous tissue in which infection shield 90 and catheter 92 mightbe implanted. Tissue cuff 94 substantially circumferentially surroundscatheter seal 96. In use, catheter seal 96 of infection shield 90substantially circumferentially surrounds a catheter 92.

Protocol for Manufacturing Porous Linked Fibrous Silica Fiber

The process for manufacturing linked silica fiber comprises preparationof a silica fiber slurry, followed by heat treatment of the slurry.Either a substantially rough or a partially smooth outer surface may beproduced on the porous linked silica fiber, depending on the heattreatment used on the slurry. A flow diagram representing the process isillustrated in FIG. 7.

In step 61, 60 g of Q-Fiber® (amorphous high purity silica fiber),Manville Division of Schuller International, Inc., Waterton, Ohio, isadded to 1000 ml of “Nyacol 1430” (colloidal silica sol), PQCorporation, Ashland, Mass. and distilled water (1 part Nyacol plus 9parts water) in a stainless steel container (“VitaMixer Maxi 4000 fromVitaMix Corporation, Cleveland, Ohio). Note that the above dilutionproduces porous linked fibrous biomaterial according to the presentinvention at a density of approximately 12 pounds per cubic foot,whereas if the silica sol is used undiluted, the density willapproximate 39 pounds per cubic foot.

In step 62, the mixture is stirred for two minutes with a rotating bladeto chop the fibers and create a homogeneous slurry. To make a linkedsilica fiber with one smooth outer surface and one rough surface, steps64, 66, 68 and 70 are executed as follows. In step 64, approximately onehundred milliliters of the slurry is poured into a Pyrex vessel (20cm×20 cm by 6 cm). Contact between the slurry and the Pyrex surface ispreferably prevented by a thin membrane placed over the Pyrex surface(e.g. Teflon). The vessel is placed in an oven at room temperature. Instep 66, the oven is heated to about 220 degrees Fahrenheit withinapproximately 5 minutes and remains at this temperature forapproximately 5 hours. In step 68, the oven temperature is then raisedto about 400 degrees Fahrenheit in approximately 10 minutes and remainsat this temperature for approximately 1 hour.

In step 70, a sheet is removed from the oven and cooled, the sheet beinga piece approximately 1 to 2 mm thick by approximately 20 cm×20 cm oflinked fiber, the piece having a bottom side (which was against thePyrex dish) that is smooth and shiny and a top side (exposed to the air)that is relatively rough. The shiny side is apparently a homogeneouslayer of deposited silica integrated into the linked fiber matting. Theshiny and rough sides are both pervious to water and hydrophilic incharacter.

To make a porous linked silica fiber with a continuous rough surfaceoverall, steps 72, 74, 76 and 78 in FIG. 7 are executed as follows. Instep 72, approximately 680 ml of the slurry prepared above in step 62 ispoured into a plastic microwaveable dish 9.5×13.5×6 cm with 12 holes0.2-0.4 cm in diameter in the bottom of the dish. The liquid of theslurry is allowed to drain through the holes over about 10 minutes. Instep 74, the fibrous mat is pressed lightly by hand using a plastic formmold piston, after which the mat is heated for 5 minutes in a microwaveoven.

In step 76, the mat is transferred in a Teflon® lined pan to an oven atapproximately 220 degrees Fahrenheit. The mat is turned over three timesevery hour. The temperature is maintained for about four and one-halfhours. In step 78, the oven temperature is raised to approximately 400degrees Fahrenheit, and the linked fiber block is removed after about 1hour and allowed to cool; all six sides of the cooled linked fiber blockare rough.

Protocol for Manufacturing Porous Fused Rigid Ceramic

One process for manufacturing fused silica/alumina and/or other ceramicfiber of low density, like 12 lb. per ft.³, comprises:

(1) preparation of a slurry mixture comprised of pre-measured amounts ofpurified fibers and deionized water;

(2) removal of shot from slurry mixture;

(3) removal of water after thorough mixing to form a soft billet;

(4) addition of a ceramic binder after the formation of the billet;

(5) placement of the billet in a drying microwave oven for moistureremoval; and

(6) sintering the dry billet in a large furnace at about 1600° F. orabove.

The high purity silica fibers above are first washed and dispersed inhydrochloric acid and/or deionized water or other solvent. The ratio ofwashing solution to fiber is between 30 to 150 parts liquid (pH 3 to 4)to 1 part fiber. Washing for 2 to 4 hours generally removes the surfacechemical contamination and non-fibrous material (shot) which wouldcontribute to silica fiber devitrification. After washing, the fibersare rinsed 3 times at approximately the same liquid to fiber ratio for10 to 15 minutes with deionized water. The pH is then about 6. Excesswater is drained off leaving a ratio of 5 to 10 parts water to 1 partfiber. During this wash and all following procedures, great care must betaken to avoid contaminating the silica fibers. The use of polyethyleneor stainless steel utensils and deionized water aids in avoiding suchcontamination. The washing procedure has little effect on the bulkchemical composition of the fiber. Its major function is theconditioning and dispersing of the silica fibers.

The alumina fibers are prepared by dispersing them in deionized water.They can be dispersed by mixing 10 to 40 parts water with 1 part fiberin a V-blender for 2½ to 5 minutes. The time required is a function ofthe fiber length and diameter. In general, the larger the fiber, themore time required.

In order to manufacture ultra low density ceramic material, for exampledensities below 12 lb/ft³ the process includes the additional steps of:

(1) the addition of expandable carbon fibers in the casting processand/or other temporary support material; and

(2) firing the billet at about 1300° F. to remove the carbon fibers orother support material prior to the final firing at approximately 1600°F. or above.

One preferred composition to practice the invention which can bemanufactured using the above method consists of the following:

(1) from about 10% to about 50% by weight alumina fiber;

(2) from about 50% to about 90% by weight silica fiber;

(3) from about 1% to about 3% by weight silicon carbide; and

(4) from about 1% to about 5% by weight boron nitride.

The preferred alumina fibers are 95.2% pure available from ICI Americas,Inc. The preferred silica fibers are 99.7% pure and are available fromManville Corp., Denver, Colo.

One preferred composition is comprised of: a ratio of silica fiber toalumina fiber of 78/22, 2% by weight 600 grit silicon carbide, and 2.85%by weight boron nitride. This composition is available commercially indensities of 3 to 12 (+/−three quarters of a pound) from LockheedMissiles and Space Co., Inc., Sunnyvale, Calif. (“Lockheed”) under thetradename “HTP” (High Temperature Performance). For example, Lockheedcommercially sells “HTP-12-22” (12 lb/ft.³ density and a silica toalumina fiber ratio of 78/22), “HTP-12-35” (12 lb/ft³ density and asilica/alumina fiber ratio of 65/35) and HTP-12-45 (12 lb./ft³ densityand silica/alumina ratio of 55/45). In addition, “HTP-6” having variousfiber ratios and a 6 lb/ft³ density is also commercially available fromLockheed.

While the above identified fibers are considered the most preferred, itshould also be noted that metal silicates, zirconia, and otherglass/ceramic fibers can also be used in the composition. Moreover,aluminaborosilicate fibers/glass can be utilized for example, Nextel312® fibers (a registered trademark of the 3M Co) can also be used inthe practice of the present invention. Nextel 312® is a fiber consistingof aluminum oxide, boria and silicon dioxide in the ratio of 3, 1, 2respectfully. The alumina burosilicate fibers should be prepared in thesame manner as the alumina fibers as set forth above.

In addition, while boron nitride is preferred, it is also believed thatSiBx, B₄C and B and other boron sources can also be used as bonding orfluxing agents. As stated, however, boron nitride is believed to bepreferred because it is believed, due to its stability, it permits amore uniform fusion to fiber junction and yields superior bonding anduniform porosity.

It should also be noted, that porous linked fibrous silica fiber(discussed in the previous section) can also be manufactured by theprocess described above for the manufacture of rigid fusedalumina/silica fibers.

According to one embodiment of the invention, 9 lbs/ft³ is the maximumdensity for mamilian cell growth. According to a further embodiment, forexample, a bioreacter, preferred density is dependent upon mean celldiameter, such that maximum cellular integration into the ceramicmaterial occurs between about 100 microns and about 1000 microns. As afurther example of a bioreactor embodiment, hepaticytes (liver cells)are grown in about five pounds per cubic foot. For a further bioreactorexample, the cell line MG63, about 6.5 pounds per cubic foot are used.As a further example, about 7.5 pounds per cubic foot is used forfibroblasts. For adipocytes, between about four and about five poundsper cubic foot is used. As yet a further bioreactor example, neuroncells are grown in a density of abotit 3 pounds per cubic foot.

According to a drug delivery embodiment, in vivo applications, densityis such that maximum tissue integration occurs to include blood vessels,nerves, and other normal organ appendages and or cell types. Further, inthe in vivo application embodiment, structural archetecture is alsoprovided for (for example, rete peg formation of squamous epithelialtissue). As a further drug delivery embodiment, in dermis for long termdrug delivery, between about six and about seven pounds per cubic footis used. As a further drug delivery embodiment, for short term drugdelivery in bone, between about four and about six pounds per cubic footis used. As yet a further embodiment, the ceramic is shaped as spheresbetween about 300 microns and about 500 microns in diameter (forexample, for BMP release in boney non-unions). As a further drugdelivery embodiment, antibiotic release into liver tissue, between aboutfour and about five pounds is used. As still a further example, forantineoplastic delivery to adipose/breast tissue, between about threeand about five pounds per cubic foot arc used.

Cleaning and Sterilization of Porous Linked Silica Fiber for CellCulture

Pieces of linked silica fiber blocks about one centimeter square by twoto three centimeters long are cut from larger blocks using a diamondblade saw cooled with distilled water. The blocks are washed twice withdistilled water and subjected to ultrasonic cleaning for three minutesin absolute ethanol in an ultrasonic bath (Transistor Ultrasonic T14,L&R). The cleaning treatment in ethanol is repeated once. The blocks aredried at 37 degrees Fahrenheit for twenty-four hours and then autoclavedfor 20 minutes at 121 degrees Centigrade and 15 psi in glass vials.

Propagation of Cells in Porous Linked Silica Fiber

Approximately 7000 cells are suspended in Dulbecco's Modified EagleMedia (GIBCO Lab, Grand Island, N.Y.) with 10% fetal calf serum. Thecells are from a human osteogenic sarcoma MG63 cell line, and arepipetted on to the upper (rough) surface of the linked silica fibersamples positioned in the center of 16 mm wells of 24-well polystyreneculture plates (Corning, Corning, N.Y.). An additional 0.5 ml of mediais added to each well. The culture plates are covered and placed in 37degree Centigrade, humidified incubators in the presence of a 5% C0₂atmosphere.

Colorimetric Assay for Cellular Growth

The method described by Mosmann (J. of Immunological Methods, 65 (1983)55-63, Rapid Colorimetric Assay for Cellular Growth and Survival:Application to Proliferation and Cytotoxicity Assays, Tim Mosmann) isused to estimate the growth of cells in the porous linked silica fiber.Briefly, MTT (3-(4,5-dimethylthiazol-2-ol)-2,5-diphenyl tetrazoliumbromide (Sigma) is dissolved in phosphate buffered saline (PBS) at 5mg/ml and filtered to sterilize. 100 ul of MTT solution is added toassay vessels and incubated three hours at 37 degrees Centigrade. Thematrix is transferred, or in the instance of wells with no matrixsample, the media and MTT solution is transferred to a centrifuge tubeinto which 2 milliliters of PBS and 1 ml of 0.04 N Hcl in isopropanol isadded. The tubes are vortexed and then incubated at room temperature for15 minutes. Two hundred and fifty microliters from each well is placedin a microfuge tube and centrifuged in a Microfuge Model 235C (AlliedFischer Scientific) for 2 minutes. Two hundred microliters istransferred to a 96 well microtiter plate. The O.D. at 600 nm ismeasured in a TiterTek MultiSkan Plus MK2 microtiter reader (Lab Systems0Y).

Results of Experiment Measuring Cell Growth on Linked Silica Fiber ofLow and High Density

Using the protocol described above, MG63 cells were incubated onQ-Fiber® linked fiber blocks for six days before harvesting andassessment of cell growth by the calorimetric assay for cell growthdetailed above. The blocks with a high density (39 pounds per cubicfoot) had low cell growth as indicated by the mean optical densityreading of 0.06. The relatively low density blocks of fused fiberceramic (12 pounds per cubic foot) supported increased growth of cellsthat resulted in production of a mean optical density reading of 0.16.There is a linear relationship between optical density reading andnumber of MG63 cells such that a colored product from MMT metabolismresults in optical density of 1.0 O.D. at 260 nm for 470,000 cells.

From these results one can conclude that for a given fibrous material,in vitro cell growth rate is substantially inversely related to densityof the material. Note that the cells placed on the high density materialfail to penetrate the material as deeply as cells placed on the lowdensity material.

Results of Experiment Measuring Cell Growth on Linked Silica Fiber ofDifferent Dimensions

The protocol described above was used except that 10,000 MG63 cells wereincubated per well. Q-Fiber® ceramic blocks of 12 pounds per cubic footof 3 mm, 6 mm and 8 mm thicknesses and 1 square centimeter wereincubated for three days. The optical density reading for the 3 mm blockwas 0.19, for the 6 mm block was 0.22 and for the 8 mm was 0.374.

From these results one can conclude that the larger the area of theblock, the greater is the cell growth rate. One would anticipate thatincreasing the size of the block will increase the capability to supportgrowth of larger numbers of cells up to the limit of the media or tissuefluid to supply nutrients within the center of the matrix.

The use of flowing media or tissue fluid moving continuously throughporous linked fibrous biomaterial to replenish nutrients and removemetabolic products in large blocks of matrices filled with cells is theessence of a continuous bioreactor. Preferred embodiments of reservoirsof the present invention which contain cultured cells function in thismanner.

What is claimed is:
 1. A porous fused fiber composition useful for theculturing of cells, said composition manufactured from alumina fibers,silica fibers, and boron nitride.
 2. The composition of claim 1 furthercomprising silicon carbide.
 3. A porous fused fiber composition usefulfor the culturing of cells, said composition manufactured from silicafibers, carbon fibers, and a fusion compound.
 4. The composition ofclaim 2 further comprising carbon fibers.
 5. The composition of claim 2having a density ranging from about 3 pounds per cubic foot to about 12pounds per cubic foot.
 6. A cell growth system for multiplying a sampleof cells comprising: a composition manufactured from silica fibers,wherein the fibers are fused together such that the compositioncomprises a plurality of voids of a predetermined mean void size; and anutrient for the cells.